Selective sorption process



June 13, 1961 E. E. SEN-SEL SELECTIVE soRPTIoN PROCESS 2 Sheets-Sheet 1Filed April ll, 1957 @if i2 aad a a 2 www 000 Zw daan. 0 www. ad 4 5,. f4 @tw 6 4 ,m Z af, /M M an m .w w

June 13, 1961 E. E. sENsEL 2,988,577

sELEcTIvE soRPTIoN PROCESS Filed April l1, 1957 2 SheebS-Sheef 2 Il IlUnited States Patent 2,988,577 SELECTIVE SORPTION VPROCESS Eugene E.Sensel, Beacon, N.Y., assignor to Texaco Inc., a corporation of DelawareFiled Apr. 11, 1957, Ser. No. 652,146 4 Claims. (Cl. 260-676) Thisinvention relates to an improved process for separating straight chainhydrocarbon from non-Straight chain hydrocarbon in a mixture thereof,and to group 2b metalcontaining zeolites effective in said process.

By straight chain hydrocarbon is meant any aliphatic or acyclic or openchain hydrocarbon which does not possess side chain branching.Representative straight chain hydrocarbons are the normal parains andthe normal olens, mono or polyoleiins, or straight chain acetylenichydrocarbons. The non-straight chain hydrocarbons comprise the aromaticand naphthenic hydrocarbons as well as the isoparaflins, isoolenichydrocarbons, and the like. Straight chain hydrocarbon-containingmixtures which are suitably treated in accordance with this inventioninclude mixed butanes, mixtures of normal alkanes and their isomers, andvarious petroleum fractions such as naphtha fraction, a gasolinefraction, a diesel oil fraction, a kerosene fraction, a gas oil fractionand the like. Particularly suitable for treatment in accordance withthis invention are straight chain hydrocarbon-containing fractionshaving a boiling point or a boiling'range in the range of 40-550" F. andcontaining a substantial amount of straight chain hydrocarbons, e.g.,2-35% by volume. More particularly, a petroleum fraction suitable foruse in practice of this invention could have an initial boiling point inthe range of 40-300 F. and an end point in the range of 15G-550 F. Apetroleum fraction for use in the practice of this invention mustcontain both straight chain and non-straight chain hydrocarbons asdemonstrated by the following composition:

Hydrocarbon type: Percent by volume Naphthenes `0-75 Aromatics 0-50Acyclic saturates (including normal paratiins and isoparafns) 2*90Acyclic unsaturates (including normal olens and isoolens) 0-5() YTypical refinery stocks or fractions which are applicable to thepractice of this invention are a wide boiling straight run naphtha, alight straight run naphtha, a heavy straight run naphtha, acatalytically cracked naphtha, a thermally cracked or thermally reformednaphtha, a catalytically reformed naphtha and the like.

Heretofore, a synthetic sodium calcium alumino-silicate, a dehydratedcrystalline zeolite, having a ratio of calcium to sodium (measured as amolecular ratio of calcium oxide to sodium oxide) between about 2:1 andabout 4:1 and designated in the trade as Linde 5A Molecular Sieve, hasbeen proposed for separating certain straight chain hydrocarbons fromnon-straight chain hydrocarbons in a gasiform mixture thereof. Broadly,the empirical formula for such sodium calcium aluminosilicate, indehydrated state, can be written (Ca, Nag) O A1203 This sorbent can bemade by exchanging calcium for some of the sodium in the sodium form ofthe type A zeolite, then removing crystal water. Properties andstructure of the type A zeolite are described in the articles of Breeket al. and Reed et al. which appear on pages 5963-5977 of the Journal ofthe American Chemical Society, No. 23, volume 78. The Iformula (lesscrystalv "'"1 Patented June 13, 1961 N312(A102)12 (SOz) 12 which is amultiple of six of the empirical mineralogical oxide formulaNazO-AIZOa'ZSiO-Z. For purposes of simplicity I prefer to use the oxidesort of formula for describing the type A zeolite structure, but it willbe understood that both kinds of formulae are interchangeable forpurposes of reference herein to zeolites of type A structure, and, wherean oxide formula concluding with Al2O3-2Si02 is used herein, thematerial being referred to is a type A zeolite.

Capacity and selectivity of said 5A Molecular Sieve for straight chainhydrocarbons are good, e.g., approximately 40-45 cc. of normal butaneper gram of this mineral sorbent at temperature of F. and pressure of760 mm. Hg as against approximately l to 3 cc. of isobutane per gram ofthe sorbent under the same conditions. At room temperature andapproximately atmospheric pressure this mineral sorbent becomes, for allpractical purposes, saturated (i.e., it has no more capacity for agaseous normal parafn such as normal butane or those of higher molecularweight) after a contact time of about 15 minutes with the straight chainhydrocarbon vapor. Furthermore, it takes almost 5 minutes to reach ofsaturation of this sorbent with normal -butane at room temperature andatmospheric pressure.

It has been proposed, for example, to separate normal butane fromisobutane as one typical operation with said 5A Molecular Sieve, and toseparate higher normal paraiiins and oleiins from non-straight chainhydrocarbons in other operations by the process which comprises:contacting the mixture of non-Straight and straight chain hydrocarbonsin vapor phase with said 5A Molecular Sieve, thereby selectively sorbingsome of the straight chain hydrocarbon; withdrawing the resultinghydrocarbon mixture depleted of straight chain hydrocarbons; anddesorbing sorbed straight chain hydrocarbon from the laden mineralsorbent to fit it for reuse. Alternating from sorption to desorption andvice-versa can be done very simply and rapidly by an essentiallyisothermal pressure swing technique. This technique is more fullydescribed hereinafter.

Efficiency of a plant yfor separation of straight from non-straighthydrocarbon using the contacting process outlined above is a function ofthe mineral sorbents selectivity for, and sorbing and desorbing ratesfor the straight chain hydrocarbons in process. This becomesparticularly evident in the instance of a xed bed contacting plantwherein a substantial shortening of the sorbing phase of the operatingcycle coupled with only a small reduction in capacity of the mineralsorbent for straight chain hydrocarbons will permit a greater number ofoperating cycles a day and, consequently, will increase the productionsignificantly.

I have now found, in a process for separating straight chain fromnon-straight chain hydrocarbons in a mixture thereof, that use of asynthetic crystalline zeolite of type A structure having about 0.3 toabout 0.95 of its exchangeable cation content as at least one lgroup 2bmetal selected from the group consisting of zinc and cadmium undercertain controlled operating conditions hereinafter described canshorten significantly the sorbing time without a proportional sacrice insorbing capacity or any significant loss in selectivity. Viewed from oneaspect, my invention permits the processing of a larger quantity ofparticular hydrocarbon mixture to desired specification with a givenweight of sorbent than has been possible heretofore. Thus, by contactingthe hydrocarbon mixture with the aforementioned group 2bmetal-containing type A zeolites under vapor phase sorbing conditions, Ifind that I can obtain 80-95% of saturation capacity of said zeolitesfor a straight chain hydrocarbon or hydrocarbons present in a very shorttime, generally in substantially less than 4 minutes, e.g., 1-3 minutes,and usually in a time as short as 1-2 minutes, or even less.

As this sorption of the straight chain hydrocarbon is occurring, thereis withdrawn from sorbing contact a hydrocarbon mixture containing areduced amount of straight chain hydrocarbon. When 80-90% saturation ofmy zeolite with sorbed hydrocarbons has been effected in the aforesaidshort time, the feed mixture of hydrocarbons is shut off, and the ladenzeolite subjected to desorbing conditions whereby previously sorbedstraight chain hydrocarbon is driven off and the zeolite made ready foranother cycle.

Not only can sorbing be made materially faster using the aboveprocessing technique and said group 2b metalcontaining type A zeolite(as compared to using the conventional sodium calcium alumino-silicate5A Molecular Sieve), but also desorbing appears to proceed at acorrespondingly faster rate under comparable desorbing conditions. Forgreatest processing throughput with a given amount of mineral sorbent,desorbing is advantageously discontinued when the zeolite containsstraight chain hydrocarbon material from previous sorbing operationsamounting to roughly -20% of saturation capacity. Thus, in preferredoperation both sorbing and desorbing is only 80-90% complete, but donevery rapidly with short contact time.

While the hydrocarbon contacting operations with my sorbents areconducted preferably as a cyclic process with a fixed bed of sorbentparticles, it is possible also to use moving or fluidized bed contact.This is particularly true when the particles of sorbent are stabilizedby methods described in the following U.S. patent applications, all ofwhich #are assigned to The Texas Company: Riordan et al., Serial No.544,244, filed on November 1, 1955; Hess et al., Serial No. 544,185, nowU.S. Patent No. 2,885,368, filed on November 1, 1955; and Ray, SerialNo. 599,231, led on July 20, 1956, now U.S. Patent No. 2,947,709.

The preferred group 2b containing sorbents of my invention are mostconveniently prepared by exchanging zinc and/ or cadmium for sodium inthe hydrated sodium form of the type A zeolite, Na2OAl2O32SiO2-4.5H2O,for example, by agitating such hydrated parent zeolite for 1/2 to 12hours in a 0.1 to 5 N aqueous zinc and/or cadmium salt solution,discarding the salt solution, and repeating the treatment with freshsolution until the necessary proportion of the sodium originally presentin the structure has been replaced by the group 2bV metal or metals.Operating at room temperature and pressure five changes of aqueous 1 Ncadmium chloride or zinc chloride are usually adequate to obtainsufiicient group 2b metal substitution for purposes of practicing myinvention. After calcining or otherwise ridding the resultant sorbent ofwater, it is receptive to straight chain hydrocarbons.

Alternatively, a hydrated sodium-calcium form of the type A zeolite,e.g., the Linde 5A Molecular Sieve, or a hydrated sodium-lithium orhydrated sodium-potassium form of the type A zeolite, can be treatedwith a zinc and/or cadmium salt solution in a similar manner to producea similarly useful type A zeolite having about 0.3-about 0.95 of itsexchangeable cation content of zinc and/or cadmium. The fraction ofexchangeable cation content referred to herein is computed as the ratioof the number of equivalents of the group 2b metal to the sum of theequivalents of all the exchangeable metals, eg., Zn++, Cd++, Na+, Li+,K+, Ca++ etc., in the resulting type A structure. i

Among the zinc salts useful in the ion exchanging are the nitrate,chloride, bromide, acetate, and sulfate. Among the cadmium salts usefulin the ion exchanging are the nitrate, chloride, bromide, sulfate,acetate, and formate. The parent sodium form of the type A zeolite canbe made by the processes shown in the following U.S. patentapplications, both of which are assigned to The Texas Company: Sensel,Serial No. 617,734, now U.S. Patent No. 2,841,471 and Estes, Serial No.617,735, now U.S. Patent No. 2,847,280, both filed on October 23, 1956.

FIGURE 1 of the drawing shows curves plotted from experimental resultsfinding the percentage of saturation (ultimate capacity) obtained withn-butane at room temperature and pressure for various contact times ofthe n-butane with two selective mineral sorbents of my invention and oneconventional selective mineral sorbent. Thus, curve A is for theconventional type A zeolite (Na2,Ca)OAl2O3-2Si02, i.e., the Linde 5AMolecular Sieve wherein the ratio of Ca to Naz was about 3:1. Curve B isfor a typical sodium-zinc form of the type A zeolite of this inventionwherein about 53% of the exchangeable cation content in the structurewas zinc. Curve C is for a typical sodium-cadmium form of type A zeoliteof this invention wherein about 40% of the exchangeable cation contentin the structure was cadmium. Inspection of the figure shows that theconventional zeolite attained only about 73% of saturation with thenormal hydrocarbon in two minutes, whereas the novel zeolites of myinvention attained about and 871/z% of saturation in two minutes.Ultimately capacity of these three mineral sorbents for n-butane underthe test conditions was practically the same.

N-butane sorbing characteristics for group 2b metalycontaining type Azeolites having broadly 0.3 to 0.95 of their exchangeable cation contentas zinc and/or cadmium and corresponding to the above test zeolites areabout the same. However, for economy and efiiciency of preparation,those having about 0.4-0.7 of the exchangeable cation content consistingof at least one group 2b metal selected from the group consisting ofzinc and cadmium are preferred.

FIGURE 2 shows curves plotted from experimental results finding straightand non-straight chain hydrocarbon capacity and selectivitycharacteristics at about room ternperature (75 F.) and atmosphericpressure for type A zeolites wherein the content of zinc in the zeolitewas varied over a wide range. As the zinc-containing type A zeolites aremade most conveniently and preferably by exchanging a portion of Na+ions for Zn++ ions in the sodium form of the type A zeolite, the testzeolites were made that Way and the x axis indicates the percentage ofsodium replaced by zinc in the type A structure. The capacities of thezinc-containing type A zeolites for the non-straight chain hydrocarbon,isobutane, and the straight chain hydrocarbons, normal butane andethane, are indicated on the y axis.

FIGURE 3 shows curves plotted from experimental results finding straightand non-straight chain hydrocarbon capacity and selectivitycharacteristics at about room temperature (75 F.) and atmosphericpressure for type A zeolites wherein the content of cadmium in thezeolite was varied over a wide range. As these cadmium-containing type Azeolites are made most conveniently and preferably by exchanging aportion of Na+ ions for Cd++ ions in the sodium form of the type Azeolite, the test zeolites were made that way and the x axis indicatesthe percentage of sodium replaced by cadmium in the type A structure.The capacities of the cadmium-containing type A zeolites for thenon-straight chain hydrocarbon, isobutane, and the straight chainhydrocarbons, normal butane and ethane, are indicated on the y axis.

While the foregoing experimental work was done mainly with butane andisobutane, it will be understood that these two hydrcarbonsare'representative of the two lvbnroadcla'sses f hydrocarbonsfiorpurposesTofthis invention, namely straight chain and nonisltraightchainhydrol carbons,4 and-r` that hydrocarbons of'y higher l; lmolecularweight, e.g., up to about V5" 7'0"vl3f.j,rnornialfboiling point, can betreated similarly except with the reservation that temperature and/ orpressure conditions must be such that the hydrocarbons are in vaporphase for sorption.

A convenient and rapid way to change from sorbing conditions todesorbing conditions in the practice of my process is to operateessentially isothermally at a temperature from about 50 to about 800 F.and to sorb under a pressure of 100 to 2000 p.s.i.g. then to desorb atlower pressure in the range from 0 to 100 p.s.i.g. or evensubatmospheric pressure. Such operation is described in United Statespatent application Serial No. 484,833 of January 28, 1955, of Hess etal., also assigned to The Texas Company.

The process of my invention can also be operated wherein temperature ofsorbing contact is between 50 and 500 F. and is raised for desorption.Alternatively, desorption can be done at a temperature substantiallyabove, and at a pressure substantially below the sorbing temperature andpressure to drive ot sorbed straight chain hydrocarbons. Desorbing canbe done advantageously by using a subatmospheric pressure, e.g., 10 to25 inches of Hg absolute, and/or a sweep of low molecular weight gas,e.g., hydrogen, nitrogen, isopentane, or methane to help drive oifdesorbed straight chain hydrocarbon vapors from the mineral sorbent.

The following examples show how type A zeolites having zinc and cadmiumas a portion of their exchangeable cation content have been prepared andhow such zeolites can be used in a refinery, but should not be construedas limiting the invention.

A type A zeolite in which 52.9% of the exchangeable cation content waszinc was made as follows: 30 grams of pelleted and dehydrated sodiumform of the type A zeolite, marketed as Linde 4A Molecular Sieve, wasallowed to soak for 36 hours at 200 F. in 70 cc. of 2.1 N aqueous ZnCl2solution. The exchanged zeolite was washed with water and dehydrated at575 F. to prepare it for sorption of straight chain hydrocarbons.

A type A zeolite in which 40.4% of the exchangeable cation content wascadmium was made as follows: 30 grams of pelleted and dehydrated sodiumform of the type A zeolite, marketed as Linde 4A Molecular Sieve, wasallowed to soak for 36 hours at 200 F. in 70 cc. of 1.5 N aqueous CdClzsolution. The exchanged zeolite was washed with water and dehydrated atabout 575 F. to prepare it .for sorption of straight chain hydrocarbons.

A cadmium-calcium-sodium form of type A zeolite was made as follows: 30grams of granular dehydrated calcium-sodium form of type A zeolite,

was allowed to soak in 72 cc. of 4.5 N aqueous CdClz solution at 200 F.for 84 hours. The granules were washed thoroughly with water anddehydrated to remove zeolitic water. Chemical analysis of the productzeolite indicated a formula (dehydrated state) as follows:

(0.62 Cd, 0.24 Ca, 0.14 Na2O)O'Al2O32SiO2 Capacity of the product at 75F. and atmospheric pressure, found by test, was 41 cc. of ethane pergram, 37 cc. of n-butane per gram, and 4 cc. of isobutane per gram.

A zinc-calcium-sodium form of the type A zeolite was made as follows: 30grams of granular dehydrated calcium-sodium for-m of type A zeolite,

was allowed to soak in 72 cc. of 4.5 N aqueous ZnCl2 solution at 200 F.for 84 hours. The granules were washed with' water and dehydrat'edftoremove zeolitic Water.: Chemical analysis of theproduct zeoliteindicated Ia formula (dehydrated state) as follows? p Capacity of theproductat"75`F. and atmospheric 'pressure, found-by test, was' 3'1"cc.of ethane' per gram," 32 cc. of n-butane per gram, and 5 cc. ofisobutane per gram.

'Ihe four foregoing products were capable of attaining from -90%saturation (ultimate capacity) with normal butane at room temperatureand atmospheric pressure in substantially less than four minutes.

IA typical hydrocarbon separation contemplated with my group 2bmetal-containing form of type A zeolite is the removal of straight chainhydrocarbons from stabilized, catalytically reformed motor naphtha, suchnaphtha having characteristics, for example, of API gravity, 48.8;refractive index about 1.444; ASTM distillation I.B.P., '126 F. and EP.,377 F.; and ASTM research clear octane rating, 87.1.

The naphtha in vapor form is passed through a first bed of my type Azeolite in pelleted form at temperature of about 750 F., and underpressure of about 350 p.s.i.g., for 2 minutes. The zeolite has 40-70% ofits exchangeable cation content a group 2b metal selected from the groupconsisting of cadmium and zinc with balance of sodium. Saturation of thepellets with straight chain hydrocarbon components is then aboutcomplete. The unsorbed naphtha vapors emerge from the bed low in thestraight chain hydrocarbon content which detracts from the octane ratingof the feed stock. At this point the naphtha feed is shunted to anothersimilar sorbent bed. A stream of recycle hydrogen from catalyticreforming of the feed naphtha is passed through the first bed, brieflyunder elevated pressure to purge the vessel of unsorbed material. Thenthe pressure on the iirst bed is reduced to 0 p.s.i.g. and the hydrogenis continued for about 2 minutes to desorb all but approximately 15-20%of the straight chain hydrocarbons in the laden sorbent pellets. Theeflluent vapors from the desorbing operation are used as part of therecycle feed to the naphtha reforming process.

I claim:

1. In a process for the separation of straight chain hydrocarbons from amixture of straight chain and' nonstraight chain hydrocarbons whereinthe mixture is contacted under vapor phase hydrocarbon sorbingconditions With a dehydrated crystalline zeolite of type A structurehaving at least a part of its exchangeable cation content replaced witha divalent metal ion for a period of time sucient to effect sorption ofsaid straight chain hydrocarbon by said zeolite, and thereafterdesorbing straight chain hydrocarbon from the laden zeolite, theimprovement which comprises contacting said mixture with a type Azeolite wherein about 0.3 to about 0.95 of the exchangeable cationcontent is divalent cadmium for a period within the range of 1 to 4minutes suicient to effect about 80 to 195 percent saturation of saidzeolite with the straight chain hydrocarbon present in the mixture.

2. A process according to claim 1 wherein from about 0.4 to about `0.7of the exchangeable cation content of the zeolite is cadmium.

'3. A process according to claim 1 wherein the contact period is from 1to 2 minutes.

4. A process according to claim 1 wherein the exchangeable cationcontent of said zeolite is cadmium, calcium and sodium.

(References on following page) P7 f" References urea in the le of thispatent -OTHER REFERENCES y Y UNITED STATES PATENTSv i Barrer et al.:Journal of the Chemeal Society, -May 1,728,732 Jaeger sept. 17, 19291952, pages 156145711- 2,306,610 Barrer Dec. 29, 1952 5 Breek et a1.:Jour. Amer. Chem. Soc., vol. 78,\No.23, 2,818,455 Ballard et al. Dec.31, 1957 pages 5963-71 (December 1956). 2,859,256 Hess et al. Nov. 4,1958 i 2,822,243 Milton Apr. 14, 1959

1. IN A PROCESS FOR THE SEPARATION OF STRAIGHT CHAIN HYDROCARBONS FROM AMIXTURE OF STRAIGHT CHAIN AND NONSTRAIGHT CHAIN HYDROCARBONS WHEREIN THEMIXTURE IS CONTACTED UNDER VAPOR PHASE HYDROCARBON SORBING CONDITIONSWITH A DEHYDRATED CRYSTALLINE ZEOLITE OF TYPE A STRUCTURE HAVING ATLEAST A PART OF ITS EXCHANGEABLE CATION CONTENT REPLACED WITH A DIVALENTMETAL ION FOR A PERIOD OF TIME SUFFICIENT TO EFFECT SORPTION OF SAIDSTRAIGHT CHAIN HYDROCARBON BY SAID ZEOLITE, AND THEREAFTER DESORBINGSTRAIGHT CHAIN HYDROCARBON FROM THE LADEN ZEOLITE, THE IMPROVEMENT WHICHCOMPRISES CONTACTING SAID MIXTURE WITH A TYPE A ZEOLITE WHEREIN ABOUT0.3 TO ABOUT 0.95 OF THE EXCHANGEABLE CATION CONTENT IS DIVALENT CADMIUMFOR A PERIOD WITHIN THE RANGE OF 1 TO 4 MINUTES SUFFICIENT TO EFFECTABOUT 80 TO 95 PERCENT SATURATION OF SAID ZEOLITE WITH THE STRAIGHTCHAIN HYDROCARBON PRESENT IN THE MIXTURE.